The present invention relates, in general, to semiconductor devices and, more particularly, to a field effect transistor for measuring a change in surface potential of the transistor's gate electrode due to exposure to a chemical as used in, for example, a chemical sensor.
Field effect transistors have been previously used in some cases as chemical sensors for measuring the concentration of a chemical in a fluid. One such prior sensor uses a gate electrode that is horizontally suspended over the channel region so as to provide a gap in which fluid may enter and contact an exposed surface of the gate electrode. A chemical in the fluid, to which the gate electrode is particularly sensitive, is adsorbed onto the exposed surface and changes the surface potential of the gate electrode. The drain current of the transistor changes in response to this surface potential change. Thus, if a constant gate voltage source is applied to the gate electrode during sensing, the change in drain current can be correlated to the concentration of the chemical in the fluid.
It has been found that the surface chemical reactions of this prior sensor, which include adsorption/desorption reactions of the chemical to be sensed onto and off of the exposed gate electrode surface, are very sensitive to temperature, so it is desirable that the temperature of the gate electrode be more directly regulated to optimize the output of the sensor. Also, it has been found to be desirable that this temperature be elevated above the ambient temperature to provide improved performance for the sensor. However, prior chemical sensors do not provide an integrated heating element for direct temperature control of the gate electrode. Instead, an external heater is required to heat the entire sensor assembly, rather than the gate electrode directly. Such an external heater is inconvenient to provide in a final, fully-manufactured chemical sensor assembly and increases the manufacturing cost thereof. Also, an external heater requires significant power consumption during operation.
In addition to the above, it is important that the gap size of the sensing transistor be readily controllable during manufacture so that the operating characteristics of the transistor are consistent over a large number of manufactured sensors. Also, it is important that the gap structure be stable under thermal loads, and the gap structure should be mechanically robust to improve reliability along with ease of handling and packaging.
Further, the mechanical structure of the transistor should permit ready diffusion of the fluid to be sensed into and out of the gap of the transistor. This ready diffusion is important to provide a faster response time of the sensor to chemical changes in the fluid. These changes cannot be fully sensed until the species corresponding to the chemical change diffuses through the gap of the transistor structure to contact the gate electrode of the sensor. Prior sensor structures do not provide a readily controllable gap size and such ready diffusion into the gap. Further, prior sensor structures need to be more compatible for integration into standard device process flows for devices for control circuitry incorporated on the same chip as the sensor.
Accordingly, there is a need for a chemical sensing field effect transistor that provides an integrated heater and a fluid gap in a transistor structure that permits improved control of the gap size, improved diffusion of a fluid into the gap, and improved compatibility with other standard device process flows.